Include custom metadata during command dispatch

[slashdotdash]

Allow metadata to be provided when dispatching a command:

command = %OpenAccount{account_number: "ACC123", initial_balance: 1_000}
metadata = %{"user_id" => "user-12345", "ip_address" => "127.0.0.1"}

:ok = BankRouter.dispatch(command, metadata: metdata)

The optional metadata is recorded against all events created by the command. This is useful for additional auditing purposes, such as recording the user's identity, their source IP address, etc.

Fixes #61.

Thanks to @sammkj for implementing this feature.